The used jumpers were described as “as worn by” or “as seen on” Prince George.

The Cath Kidston spokesman said: “Our guards tank top, as seen on Prince George is now sold out. Due to popular demand, we are going to look at adding it back into our range of kidswear in 2015.”

The spokesman added that similar tank tops were available in-store and online.

In the past, parents wanting to dress their children like the baby prince have resorted to fierce bidding wars on eBay, where items have fetched beyond the normal retail price.

Designers have also sold out of stock worn by the third-in-line to the throne within hours of him being seen in it.

When he was pictured wearing a blue jumper personalised with his name, baby retailer My1stYears.com sold out of a similar item it stocked within four days.

The “George effect” was coined shortly after the prince was born when sales of the swaddling blanket he was wrapped in as he left hospital rose by 600% in the following week.

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A pair of £39.99 red-striped dungarees by Spanish children's wear label Neck & Neck worn by George on Father’s Day sold out within 24 hours of him being seen in them.

Dungarees from London-based children’s brand Rachel Riley he wore in New Zealand during his first official engagement were equally popular. Ms Riley said the royal endorsement had been like hitting the jackpot for the business.

His mother, the Duchess of Cambridge, has had the same effect with her fashion choices.

The “Kate effect” has seen fans rush to the duchess's favourite high street stores like Zara and LK Bennett and Reiss in the hopes of emulating her style.